Joint compound preparation is described in U.S. Pat. No. 891,582. Assignee's U.S. Pat. application, Ser. No. 625,999, discloses the use of salt suspensions to prepare aqueous joint compounds.
Gypsum wallboard has displaced plaster in the erection of interior walls in the construction industry, especially in the United States. Wallboard is generally installed in large panels which are nailed and glued to the studs of the wall and fitted together until the entire section of the wall is covered. The joints, where sections of the board are butted together, are covered with joint compound, then with joint tape, and then the taped joints and all nails are covered with a joint compound which, upon hardening, can be sanded and smoothed so that it is imperceptible under paint, wallpaper or textured coating.
Joint compounds used with wallboard usually contain a resinous binder, limestone, clay, mica, lubricant, stabilizer and a thickener as the principle ingredients. These are mixed with water to form a homogeneous paste which is normally applied with a trowel.
Typically, the water soluble polymer used to thicken the joint compound is a cellulose ether, e.g., hydroxyethylcellulose (HEC), hydroxypropylmethylcellulose (HPMC), etc. One of the factors limiting the output of large quantities of a joint compound in a commercial plant is the relatively long time necessary to completely disperse and dissolve the water soluble polymer thickener. Because of this, the ingredients are usually mixed for times ranging from 20 to 60 minutes. The water soluble polymer may be dry blended with the other ingredients, or added as an aqueous slurry. In the latter case, the hydration rate of the water soluble polymer must be chemically retarded so that the slurry does not thicken prior to being added to the remaining ingredients. The slurry has a finite lifetime, measured in minutes, even when a chemical retarder is used to temporarily crosslink the polymer and delay the onset of hydration.
There are other problems associated with the manufacture of joint compounds. These are of both a technical and practical nature.
When a dry thickener is employed, problems arise with dustiness (health and potential explosion hazard); caking and lumping (hydrophilicity of the thickener); broken bags (waste and slipping hazards); messy storage area, bag disposal and inconvenience. The construction industry would welcome the elimination of any of the above.
The use of dry or state of the art thickeners results in a long production, time which is needed to properly disperse and dissolve the thickener. It also results in high energy consumption because of high torque required to mix the thickener with the other ingredients and in the long mixing time needed to dissolve and disperse the thickener.